Biochemical and biophysical pH clamp controlling Net H⁺ efflux across the plasma membrane of plant cells

Abstract

Summary P‐type H+ ATPases mediate active H+ efflux from plant cells. They generate a proton motive force across the plasma membrane, providing the free energy to drive the transport of other solutes, partly by coupling to H+ influx. Wegner & Shabala (2020) recently suggested that passive H+ influx can exceed pump‐driven efflux due to ‘active buffering’, that is, cytosolic H+ scavenging and apoplastic H+ generation by metabolism (‘biochemical pH clamp’). Charge balance is provided by K+ efflux or anion influx. Here, this hypothesis is extended to net H+ efflux: even though H+ pumping is faster than backflow via symporters and antiporters, a progressive increase in the transmembrane pH gradient is avoided. Cytosolic H+ release is associated with bicarbonate formation from CO2. Bicarbonate serves as substrate for the PEPCase, catalyzing the reaction from phosphoenolpyruvate to oxaloacetate, which is subsequently reduced to malate. Organic anions such as malate and citrate are released across the plasma membrane and are (partly) protonated in the apoplast, thus limiting pump‐induced acidification. Moreover, a ‘biophysical pH clamp’ is introduced, that is, adjustment of apoplastic/cytosolic pH involving net H+ fluxes across the plasma membrane, while the gradient between compartments is maintained. The clamps are not mutually exclusive but are likely to coexist.

Genomically Hardwired Regulation of Gene Activity Orchestrates Cellular Iron Homeostasis in Arabidopsis

“…Another peculiarity associated with the metabolism of Fe-deficient root cells is related to the massively increased export of protons, which necessitates strategies to replenish substrate for the H + -ATPases and to prevent excessive alkalisation of the cytosol. The concentration of free cytosolic H + is in the sub-micromolar range [ 71 ] and insufficient to support the high proton fluxes of Fe-deficient root cells. Such compensatory release of protons is achieved by PPC-mediated carboxylation of PEP or, more precisely, the preceding formation of HCO 3 − by carbonic anhydrase, which is accompanied by a net production of protons ( Figure 11 ).…”

Genomically hardwired regulation of gene activity orchestrates cellular iron homeostasis in Arabidopsis

“…The product of PEP carboxylation, OAA, was not detectable in roots, indicative of its rapid metabolization to citrate, the level of which was dramatically increased upon Fe starvation. Citrate accumulation in roots of Fe-deficient has been reported for a variety of species with high proton extrusion capacity such as tomato 16 , cucumber 71, 72 , sugar beet 73 , and Capsicum annuum 74 , implying a link between these two observations. Moreover, citrate levels appear to correlate with PPC activity 75 , making it tempting to speculate that increased proton secretion and subsequent PEP carboxylation are the driving forces for citrate accumulation in Fe-deficient roots.…”

Transporter networks can serve plant cells as nutrient sensors and mimic transceptor-like behavior

“…As pointed out by Lars Wegner et al. ( Wegner et al., 2021 ; Wegner and Shabala, 2020 ), H + scavenging is mainly associated with malate decarboxylation catalyzed by malic enzyme, and via the GABA shunt of the tricarboxylic acid (TCA) cycle involving glutamate decarboxylation. We simulated different cellular buffer capacities by different starting values for [ Buf – ] and [ HBuf ].…”